1. Field of the Invention
The present invention relates to an amplifier, which especially handles an analog vector signal such as a complex number signal, and a filter and a radio communication device using the same.
2. Description of the Related Art
In a radio communication field as of 2005, there are adopted many systems for modulating both an amplitude and a phase of a signal. Therefore, in many cases, two orthogonal signals are used which are referred to as an in-phase signal (I-signal) and a quadrature-phase signal (Q-signal).
For example, in a case where a filter is formed, for example, on an integrated circuit as of around 1995, a single-ended system (circuit system in which a voltage between a signal line and the ground is handled as the signal) is adopted for both the I-signal and the Q-signal. An example of a filter of the single-ended system is described in “A 2.5-V active low-pass filter using all-n-p-n Gilbert cells with a 1-Vp-p range”, authored by M. Koyama, T. Arai, H. Tanimoto, and Y. Yoshida, IEEE Journal of Solid-State Circuits, Vol. 28, No. 12, December 1993, pp. 1246 to 1253.
Since 2000, there has been increasingly adopted a differential system (circuit system in which a voltage between a positive signal line and a negative signal line is handled as the signal) for both the I-signal and the Q-signal. An example of a filter of the differential system is disclosed in “A 2.7-V, 200-kHz, 49-dBm, stopband-IIP3, low-noise, fully balanced gm-C filter IC”, authored by T. Itakura, T. Ueno, H. Tanimoto, A. Yasuda, R. Fujimoto, T. Arai, and H. Kokatsu, IEEE Journal of Solid-State Circuits, Vol. 34, No. 8, August 1999, pp. 1155 to 1159.
An advantage of the single-ended system is that components are less than those of the differential system. Since a communication system in the 1990s has a low transmission rate, and requires a plurality of capacitors, several capacitors are mounted as external components of the integrated circuit on a circuit board. When the external components are as few as possible, costs are reduced. Therefore, the single-ended system is preferable.
However, in the single-ended system, separately from a power voltage and a ground potential, an analog ground potential needs to be supplied as an analog reference voltage to each amplifier in the integrated circuit. In this case, since a signal current flows into the analog ground potential, it is necessary to use an analog ground buffer amplifier having a high current driving capability (current supply capability and/or current absorbing capability). When an output impedance of the buffer amplifier is high, the analog ground potential varies with the signal current. This variation causes, for example, signal leakage from the I-signal into the Q-signal, or signal leakage from output into input. The latter signal leakage causes a problem of oscillation of the circuit.
Since 2000, in a radio communication system, a band has been broadened for high-speed data transmission. Therefore, a capacitor having a comparatively small capacity is being used. Most of components can be integrated on a chip, and accordingly the differential system has been increasingly adopted. In the differential system, a positive terminal and a negative terminal are loaded with voltages having an equal size and reverse polarities, respectively, to perform amplification or the like. An average value of the voltages of the positive and negative terminals virtually plays a role of the analog ground potential. Since the current output from the positive terminal flows into the negative terminal, no analog ground terminal has to be prepared. Since the differential system does not require the analog ground buffer amplifier required for the single-ended system, power consumption of the differential system becomes smaller than that of the single-ended system. Therefore, at present, the differential system is adopted in most cases.
As the cost per unit area of a semiconductor chip rises with development of a semiconductor fine processing technology, an analog circuit, especially, a passive element in the analog circuit occupies a large ratio of a chip area. Therefore, reduction of the area occupied by the passive element in the semiconductor chip is an important problem for cost reduction.
The single-ended system is advantageous for the reductions of the chip area and the cost in that there are fewer components. On the other hand, in the single-ended system, since the analog ground potential is supplied to each circuit block as described above, the analog ground buffer amplifier having a high current driving capability is required, and therefore, power consumption increases.
The differential system does not require the analog ground buffer amplifier required for the single-ended system, and the differential system has an advantage that the power consumption is reduced. On the other hand, the components of the differential system are more than those of the single-ended system, and the cost increases in the differential system.